Why do we have hair?
...If hairs be wires, black wires grow on her head.
(Sonnet 130, WS)
We [the mammals] have hair; it is our definitive characteristic, nobody else has it. Some of us - including whales and dolphins - have very little of it, but we all do. "Hairless" humans have as many hairs on their bodies as apes do, but ours are much thinner. Hair insulates, it protects against the sun, hair enhances the tactile sense, it is ornamental, and it acts to communicate emotion, by piloerection. How did we get hair?
It is the usual problem: the adaptation that makes no obvious sense unless it goes all the way. The usually given and obviously incorrect answer is that hair evolved to help endothermy (active maintenance of body temperature). Of course, nobody has any idea what warm blood was good for in the proto-mammals, but with hair it is worse, because one needs lots of hair (fur) for these hair to be of much use for thermoregulation. Furthermore, as we all know, hair is redundant for keeping body warmth (subcutaneous fat performs equally well and mammals, unlike flying brids, can afford it). In any case, keeping warmth during the warmest period in 500 Myr (when the mammals evolved) was hardly a problem. On the other hand, hair is the perfect home for insect parasites. The insulation works both ways, preventing efficient passive thermoregulation (basking in the sun), so it provides disadvantage to an ectothermic proto-mammal whereas great advantage comes only after well-developed endothermy. The original function of hair cannot be thermal insulation, sun screening, or any other function that required high density or regulation of endothermy. What was this advantage?
From the biochemist's point of view, mammalian skin and hair are unbelievably retrograde, like many other mammalian traits, e.g., peeing. These are not even reptilian traits; one has to go back to the amphibians in order to find the closest analogy among the living animals. The main body proteins in land animals are those that make connective tissue: collagens and keratins (50% of all proteins). Because collagens are made in huge quantity, these consist mainly of the two simplest amino acids, glycine and alanine. Keratin is like collagen but it also includes serine, glutamic acid and 10-15% of cysteine; the -SH termini of the cysteine residues in peptide chains link together and that gives keratin fibers their rigidity. The unpleasant smell of burnt hair and horn is from the sulfur in these disulfide bridges. Highly repetitive structural proteins come in two basic shapes: alpha-helices and beta-sheets. In the helices, there are H-bonds through each helix; in the beta sheets, the bonds are between two straight peptide chains. In general, the sheets are tougher than the helices. In mammals and amphibians, the keratin is alpha; in reptiles and birds the keratin is beta. We compensate for the deficient form of keratin by having more S-S bridges than absolutely necessary, and the highest fraction of such bridges is in hair. The interesting property of alpha-keratin is that, as it is a cross-linked helical protein, it can potentially make fibrous hair. Vertebrate beta-keratins cannot do that (although arthropod fibroins can, e.g. silk) making instead rigid lamellar structures, like feathers and scales. Before going any further, it is useful to remind ourselves who we are:
...The evolution of mammals from synapsids (mammal-like "reptiles") was a gradual process which took approximately 70 Myr, beginning in the mid-Permian. By the mid-Triassic, there were many species that looked like mammals, and the first true mammals appeared in the early Jurassic. The earliest known marsupial, Sinodelphys, appeared 125 Mya in the early Cretaceous, around the same time as Eomaia, the first known eutherian (member of placentals' "parent" group); and the earliest known monotreme, Teinolophos, appeared two million years later. Mammals are the only surviving synapsids. The synapsid lineage became distinct from the sauropsid ("reptile") lineage in the late Carboniferous period, between 320 and 315 Mya, and were the most common and largest land vertebrates of the Permian period. In the Triassic period a previously obscure group of sauropsids, the archosaurs, became the dominant vertebrates and one archosaur group, the dinosaurs, dominated the rest of the Mesozoic era. These changes forced the Mesozoic mammaliforms into nocturnal niches, and may have contributed greatly to the development of mammalian traits such as endothermy, hair and a large brain. (Wiki)
We are the sole surviving descendants of the evolutionary dead end, the backward synapsids that were superseded by the advanced sauropsids. One of these many advances was the development of beta-keratin that allowed an elegant solution to the design of elastic, protective skin that also prevented excessive water loss: reptilian scaly skin. The synapsids stuck to the alpha-keratin of their amphibian ancestors. Perhaps they also had scales for protecting their skin, but they also had something that was unique to them, due to their very backwardness: hair. The best idea I've seen is that the hair originated as purely sensory appendage (like cat's whiskers) in the synapsids, and then it became involved in passive thermoregulation and skin protection after a freak mutation that increased hair density. Hair is not a mammalian trait at all, it is a synapsid trait, and the evolution of hair occurred not in mammals, but in the theropsid ancestors of the synapsids and the sauropsids. Hair and hard scales are two different solutions to the problem of skin abrasion, water loss, and passive thermoregulation that are based on two different biochemistries, one archaic (alpha-keratins), another modern (beta-keratins). Millions of years later, hair (combined with endothermy) allowed nocturnal way of life under the iron heal of the dinosaurs. The descendants of the furry synapsids were on the way of becoming true mammals.
...A 1972 model suggested a primary mechanoreceptor role improving behavioral thermoregulation contributed to the success of late Paleozoic mammal-like reptiles. An insulatory role appeared secondarily subsequent to protohair multiplication. Multiplication of sensory protohairs caused by mutations in patterning genes initially protected the delicate barrier tissues and eventually produced the minimal morphology necessary for an insulatory pelage. The latter permitted Mesozoic mammals to occupy the nocturnal niche 'in the shadow of dinosaurs'.
...A plausible explanation for scale reduction and loss in early synapsids is that Theropsid amniotes pursued a different strategy to cope with environmental abrasion of their skin than that seen in other reptiles (Sauropsids) although, interestingly, birds 'copied' this strategy when feathers evolved. The nature of, and basis for, the hypothesized theropsid strategy can be summarized. A mutation involving a molecular trigger involved in patterning produced the initial multiplication of mechanosensory 'protohairs' and the resultant 'protopelage' had an initial selective advantage in that it was the final step in providing a tissue to protect the barrier to CWL. Later, as endothermy was gradually perfected in early mammals, perhaps a Jurassic event when they occupied the nocturnal niche, an insulatory boundary was further improved by multiplication of non-tactile hairs.
...When and how did amniotes acquire a barrier to cutaneous water loss (CWL)? In amphibians, enhancement of alpha-keratogenicity (primitively represented in all fish by 70 Ang tonofilaments) by a 'cell envelope' (marginal layer containing involucrin) permitted formation of a stratified, cornified epidermis. In Palaeozoic, anthracosaurian amphibians, or 'Basal Amniotes', alpha-keratinization was enhanced by filaggrin derived from a primordial histidine rich protein (HRP). Lipogenic lamellar bodies might have provided some barrier to CWL, but the 'delicate' epidermal tissues covering scales were susceptible to abrasion so that organisms were still restricted to a 'wet' aquatic environment.
...In Sauropsids, mechanical protection derives from the beta-keratogenic epidermal tissues covering reptilian scales, or avian feathers. In Sauropsids, the barrier to CWL is housed in tissues that are protected by overlying beta-keratogenic tissues. Their various degrees of efficacy permitted exploitation of desiccating and abrasive terrestrial niches.
...Early Theropsid Amniotes evolved a skin structurally and functionally similar to that of modern toads, living amphibians whose reduced epidermal mucogenicity and cornification permits them to walk over dusty driveways and hide in garages! Theropsids lost scales because their alpha-keratogenic epidermis was somewhat toughened by mammalian-type HRP, and the derived hydrophilic filaggrin facilitates epidermal flexibility. Their barrier tissues, whose minimal efficacy is suggested by the environments they inhabited, remained susceptible to abrasion unless 'lubricated' regularly. These organisms almost certainly practiced 'behavioral thermoregulation' as do many living frogs and toads, and this behavior was facilitated by the spatially patterned sensory protohairs.
...In late Theropsid Amniotes, multiplication of protohairs, caused perhaps by a mutation leading to up-regulation of a patterning trigger such as beta-catenin, provided the necessary enhanced mechanical protection for the thin stratum corneum. Once the included barrier tissues were less susceptible to abrasion, exploration of 'true terrestrial' niches became possible. In basal Mammals, the 'protopelage' - whose constituent units might have been strengthened by trichohyalin, to form 'true hairs' - had dual roles. First, an increased density of hairs further improved mechanical protection of inter-follicular barrier tissues. Second, this density also permitted an insulatory function that could have been further enhanced by other changes in the patterning mechanisms.
http://pt.wkhealth.com/pt/re/exde/pdfhandler.00023629-200306000-00001.pdf
also http://icb.oxfordjournals.org/cgi/content/abstract/12/1/159
This is a pretty story, but it is only a story. Why do we have hair?
(Sonnet 130, WS)
We [the mammals] have hair; it is our definitive characteristic, nobody else has it. Some of us - including whales and dolphins - have very little of it, but we all do. "Hairless" humans have as many hairs on their bodies as apes do, but ours are much thinner. Hair insulates, it protects against the sun, hair enhances the tactile sense, it is ornamental, and it acts to communicate emotion, by piloerection. How did we get hair?
It is the usual problem: the adaptation that makes no obvious sense unless it goes all the way. The usually given and obviously incorrect answer is that hair evolved to help endothermy (active maintenance of body temperature). Of course, nobody has any idea what warm blood was good for in the proto-mammals, but with hair it is worse, because one needs lots of hair (fur) for these hair to be of much use for thermoregulation. Furthermore, as we all know, hair is redundant for keeping body warmth (subcutaneous fat performs equally well and mammals, unlike flying brids, can afford it). In any case, keeping warmth during the warmest period in 500 Myr (when the mammals evolved) was hardly a problem. On the other hand, hair is the perfect home for insect parasites. The insulation works both ways, preventing efficient passive thermoregulation (basking in the sun), so it provides disadvantage to an ectothermic proto-mammal whereas great advantage comes only after well-developed endothermy. The original function of hair cannot be thermal insulation, sun screening, or any other function that required high density or regulation of endothermy. What was this advantage?
From the biochemist's point of view, mammalian skin and hair are unbelievably retrograde, like many other mammalian traits, e.g., peeing. These are not even reptilian traits; one has to go back to the amphibians in order to find the closest analogy among the living animals. The main body proteins in land animals are those that make connective tissue: collagens and keratins (50% of all proteins). Because collagens are made in huge quantity, these consist mainly of the two simplest amino acids, glycine and alanine. Keratin is like collagen but it also includes serine, glutamic acid and 10-15% of cysteine; the -SH termini of the cysteine residues in peptide chains link together and that gives keratin fibers their rigidity. The unpleasant smell of burnt hair and horn is from the sulfur in these disulfide bridges. Highly repetitive structural proteins come in two basic shapes: alpha-helices and beta-sheets. In the helices, there are H-bonds through each helix; in the beta sheets, the bonds are between two straight peptide chains. In general, the sheets are tougher than the helices. In mammals and amphibians, the keratin is alpha; in reptiles and birds the keratin is beta. We compensate for the deficient form of keratin by having more S-S bridges than absolutely necessary, and the highest fraction of such bridges is in hair. The interesting property of alpha-keratin is that, as it is a cross-linked helical protein, it can potentially make fibrous hair. Vertebrate beta-keratins cannot do that (although arthropod fibroins can, e.g. silk) making instead rigid lamellar structures, like feathers and scales. Before going any further, it is useful to remind ourselves who we are:
...The evolution of mammals from synapsids (mammal-like "reptiles") was a gradual process which took approximately 70 Myr, beginning in the mid-Permian. By the mid-Triassic, there were many species that looked like mammals, and the first true mammals appeared in the early Jurassic. The earliest known marsupial, Sinodelphys, appeared 125 Mya in the early Cretaceous, around the same time as Eomaia, the first known eutherian (member of placentals' "parent" group); and the earliest known monotreme, Teinolophos, appeared two million years later. Mammals are the only surviving synapsids. The synapsid lineage became distinct from the sauropsid ("reptile") lineage in the late Carboniferous period, between 320 and 315 Mya, and were the most common and largest land vertebrates of the Permian period. In the Triassic period a previously obscure group of sauropsids, the archosaurs, became the dominant vertebrates and one archosaur group, the dinosaurs, dominated the rest of the Mesozoic era. These changes forced the Mesozoic mammaliforms into nocturnal niches, and may have contributed greatly to the development of mammalian traits such as endothermy, hair and a large brain. (Wiki)
We are the sole surviving descendants of the evolutionary dead end, the backward synapsids that were superseded by the advanced sauropsids. One of these many advances was the development of beta-keratin that allowed an elegant solution to the design of elastic, protective skin that also prevented excessive water loss: reptilian scaly skin. The synapsids stuck to the alpha-keratin of their amphibian ancestors. Perhaps they also had scales for protecting their skin, but they also had something that was unique to them, due to their very backwardness: hair. The best idea I've seen is that the hair originated as purely sensory appendage (like cat's whiskers) in the synapsids, and then it became involved in passive thermoregulation and skin protection after a freak mutation that increased hair density. Hair is not a mammalian trait at all, it is a synapsid trait, and the evolution of hair occurred not in mammals, but in the theropsid ancestors of the synapsids and the sauropsids. Hair and hard scales are two different solutions to the problem of skin abrasion, water loss, and passive thermoregulation that are based on two different biochemistries, one archaic (alpha-keratins), another modern (beta-keratins). Millions of years later, hair (combined with endothermy) allowed nocturnal way of life under the iron heal of the dinosaurs. The descendants of the furry synapsids were on the way of becoming true mammals.
...A 1972 model suggested a primary mechanoreceptor role improving behavioral thermoregulation contributed to the success of late Paleozoic mammal-like reptiles. An insulatory role appeared secondarily subsequent to protohair multiplication. Multiplication of sensory protohairs caused by mutations in patterning genes initially protected the delicate barrier tissues and eventually produced the minimal morphology necessary for an insulatory pelage. The latter permitted Mesozoic mammals to occupy the nocturnal niche 'in the shadow of dinosaurs'.
...A plausible explanation for scale reduction and loss in early synapsids is that Theropsid amniotes pursued a different strategy to cope with environmental abrasion of their skin than that seen in other reptiles (Sauropsids) although, interestingly, birds 'copied' this strategy when feathers evolved. The nature of, and basis for, the hypothesized theropsid strategy can be summarized. A mutation involving a molecular trigger involved in patterning produced the initial multiplication of mechanosensory 'protohairs' and the resultant 'protopelage' had an initial selective advantage in that it was the final step in providing a tissue to protect the barrier to CWL. Later, as endothermy was gradually perfected in early mammals, perhaps a Jurassic event when they occupied the nocturnal niche, an insulatory boundary was further improved by multiplication of non-tactile hairs.
...When and how did amniotes acquire a barrier to cutaneous water loss (CWL)? In amphibians, enhancement of alpha-keratogenicity (primitively represented in all fish by 70 Ang tonofilaments) by a 'cell envelope' (marginal layer containing involucrin) permitted formation of a stratified, cornified epidermis. In Palaeozoic, anthracosaurian amphibians, or 'Basal Amniotes', alpha-keratinization was enhanced by filaggrin derived from a primordial histidine rich protein (HRP). Lipogenic lamellar bodies might have provided some barrier to CWL, but the 'delicate' epidermal tissues covering scales were susceptible to abrasion so that organisms were still restricted to a 'wet' aquatic environment.
...In Sauropsids, mechanical protection derives from the beta-keratogenic epidermal tissues covering reptilian scales, or avian feathers. In Sauropsids, the barrier to CWL is housed in tissues that are protected by overlying beta-keratogenic tissues. Their various degrees of efficacy permitted exploitation of desiccating and abrasive terrestrial niches.
...Early Theropsid Amniotes evolved a skin structurally and functionally similar to that of modern toads, living amphibians whose reduced epidermal mucogenicity and cornification permits them to walk over dusty driveways and hide in garages! Theropsids lost scales because their alpha-keratogenic epidermis was somewhat toughened by mammalian-type HRP, and the derived hydrophilic filaggrin facilitates epidermal flexibility. Their barrier tissues, whose minimal efficacy is suggested by the environments they inhabited, remained susceptible to abrasion unless 'lubricated' regularly. These organisms almost certainly practiced 'behavioral thermoregulation' as do many living frogs and toads, and this behavior was facilitated by the spatially patterned sensory protohairs.
...In late Theropsid Amniotes, multiplication of protohairs, caused perhaps by a mutation leading to up-regulation of a patterning trigger such as beta-catenin, provided the necessary enhanced mechanical protection for the thin stratum corneum. Once the included barrier tissues were less susceptible to abrasion, exploration of 'true terrestrial' niches became possible. In basal Mammals, the 'protopelage' - whose constituent units might have been strengthened by trichohyalin, to form 'true hairs' - had dual roles. First, an increased density of hairs further improved mechanical protection of inter-follicular barrier tissues. Second, this density also permitted an insulatory function that could have been further enhanced by other changes in the patterning mechanisms.
http://pt.wkhealth.com/pt/re/exde/pdfhandler.00023629-200306000-00001.pdf
also http://icb.oxfordjournals.org/cgi/content/abstract/12/1/159
This is a pretty story, but it is only a story. Why do we have hair?